The present invention relates to a system for thermally insulating tubular bodies, for example pipes for transporting cold or hot fluids.
Many kinds of thermal insulating systems are known. In particular, it is known that in order to form the lagging of a body of any shape it is possible to provide such a body with a double outer wall, in the interspace of which can be placed a material having low thermal conductivity, such as mineral wool, glass wool or polyurethane.
However, the insulating properties of such materials are not very high, and in some cases it is necessary to use great thicknesses thereof for maintaining the internal temperature of the body constant. This is, for instance, the case with undersea pipes for transporting crude oil, that are generally formed by two coaxial tubes of carbon steel or stainless steel, in which the oil flows in inner tube, while the outer tube acts as a protection. This construction is known in the field as “pipe-in-pipe.” In order to allow long-distance piping of oil, while avoiding increases in its viscosity, the oil must be maintained at the lifting temperature between about 25 and 90° C. Therefore, in the interspace between the two tubes a great amount of insulating material must be inserted. This requires the use of an over-sized outer tube, and consequently the overall volume and weight of the pipe increase notably, since the amount of steel required for the outer tube rises quickly as a function of the diameter thereof. Also, the costs for producing the pipe increase proportionally.
Alternatively, the interspace between the coaxial tubes may be evacuated, so as to exploit the low thermal conductivity of a vacuum with a view to achieving insulation of the pipe. In this case, however, the construction process of the pipe becomes more complex, and it is necessary to place in the same interspace a getter material able to absorb the gases that over time may outgas from the steel forming both tubes.
There are further well known evacuated insulating panels formed by an envelope, wherein a filling material is present under vacuum. The envelope serves to prevent (or reduce to the highest degree) the entrance of atmospheric gases into the panel, so as to maintain a vacuum level that is compatible with the thermal insulation degree required by the application. To this end, the envelope is made of so-called “barrier” sheets, which are flexible sheets characterized by a low gas permeability. Barrier sheets can be formed of a single component, generally polymeric, such as polyolefin or polyester (e.g., polyethylene terephthalate, PET). More commonly, however, barrier sheets are multilayers of different components. In the case of multilayers, the “barrier” effect is given by one of the component layers (this may be a polymeric layer, an aluminum foil, or a metallized plastic layer), whereas the other layers generally have the function of mechanically supporting and protecting the barrier layer. Multilayer barrier sheets are described, e.g., in U.S. Pat. Nos. 4,594,279; 5,142,842; 5,236,758; and 5,943,876. The filling material, on the contrary, mainly has the function of maintaining the spacing of the opposing faces of the envelope when a vacuum is formed in the panel. The filling material must have a porous or uneven internal structure, so that the pores or spaces thereof may be evacuated to perform the insulating function. This material can be inorganic, for example silica powder, glass fibers, aerogels, diatomaceous earth, etc.; or polymeric, such as polyurethane or polystyrene rigid foams, both in the form of boards and powders.
Thanks to their very low thermal conductivity, relatively thin evacuated panels are adequate to carry out an effective insulation of oil ducts. Therefore, it is possible to reduce the internal dimensions of the interspace of such ducts, thus alleviating the above named problems.
For example, International patent publication No. WO 01/38779 describes an evacuated insulating panel having a tubular shape and suitable to be placed within the interspace of an undersea conduit for oil piping.
However, a first inconvenience of such panels is the brittleness of their envelope, which can easily crack and may thus allow the passage of gases into the panel. Such a passage obviously jeopardizes the insulating properties of the panel and, in the case of undersea pipelines, it causes an irreparable damage, because the replacement of the damaged panel cannot be effected.
Another drawback of evacuated panels lies in that they do not provide an adequate insulation to tubular bodies. As a matter of fact, they generally have a planar shape and must therefore be bent to abut two opposed edges, in order to fit them to the tubular form of the inner interspace of oil ducts.
However, an evacuated panel curved in this manner does not allow perfect insulation of the inner tube of the pipe, and in particular the zone corresponding to the edges that are abutting may become poorly insulated. In that zone, in particular, a cooling of the inner tube can occur, and consequently also the oil flowing in the inner tube becomes cold, thus thickening and causing a partial obstruction in the pipe.
U.S. Pat. No. 6,110,310 describes a system for the thermal insulation of pipe-in-pipe conduits, formed of at least two layers of superimposed curved insulating panels. The joints of the panels are preferably staggered, so that there is almost no part of the inner pipe that “sees” the outer pipe, resulting in a further reduction in heat loss. However, the panels of U.S. Pat. No. 6,110,310 contain as filler a molded element made of microporous materials. The microporous materials comprise a mixture of inorganic oxides and preferably also inorganic fibers, to increase the mechanical stability of the molded element. The molded elements of this patent require rather deep incisions in order to be bent, making their construction rather complex. Besides, inorganic filler materials are rather heavy.